PROJECT SUMMARY Two thirds of the US population are overweight or obese. Stably lowering the body weight is highly challenging due to a compensatory homeostatic mechanism - metabolic adaptation - which lowers the body?s metabolic rate to prevent further weight loss. Preventing or reversing metabolic adaptation would provide a powerful new treatment for obesity; however, the molecular and neuronal basis of metabolic adaptation and body weight homeostasis remain poorly understood. Calorically restricted (CR) mice display decreased metabolic rates, suggesting that mice could be used as a model to study metabolic adaptation. Using a CR mouse model, this proposal aims to investigate the neuronal basis of metabolic adaptation. In preliminary studies, the applicant used recently developed genetic tools to label neurons, across the brain, that are active during caloric restriction. Subsequent chemogenetic reactivation of these neurons in a non-fasted state was sufficient to lower the metabolic rate suggesting that this strategy successfully captured, or TRAPed, a population of metabolic- adaptation-regulating (MAR) neurons. The goal of this proposal is to first identify and then molecularly and physiologically characterize MAR neurons during calorie restriction to shed light on the mechanism by which they induce metabolic adaptation. This proposal contains a comprehensive training and research plan to build upon these initial findings by characterizing these neurons in the short-term, and by facilitating the establishment of an innovative and multidisciplinary research program focused on studying energy balance and body weight homeostasis in the long-term. In Aim 1, the applicant will complete the preliminary functional screen across brain regions to identify the anatomical location of MAR neurons and test whether their activity is both sufficient and necessary for metabolic adaptation to caloric restriction. In Aim 2, the applicant will learn and apply tools to map the synaptic inputs and targets of MAR neurons to identify new components of the metabolic adaptation circuit. In Aim 3, the applicant will learn and apply techniques to monitor neuronal activity in awake, behaving CR animals to determine the information encoded by MAR neurons and generate hypotheses regarding their function in the body weight homeostasis feedback circuit. Finally, in Aim 4, the applicant will use single-cell transcriptomics to molecularly identify MAR neurons, enabling the generation of cell-type-specific reagents and the discovery of new targets to modulate their activity and prevent or reverse metabolic adaptation during weight loss. Specific technical training activities to investigate neuron?s activity patterns and map their synaptic connections will be augmented by focused mentorship from several highly successful scientists who are committed to aiding in the applicant?s acquisition of professional and intellectual skills in the highly energetic and collaborative training environment at Harvard Medical School. This intensive career development plan is designed to facilitate the successful launch of the applicant?s independent research career, focused on studying energy balance at an academic institution.